Limited information is available regarding the frequency, spectrum, and clinical relevance of somatic mutations in the developing fetus. The goal of this study was to determine somatic mutant frequencies (Mfs) at the hypoxanthine phosphoribosyltransferase (HPRT) reporter gene in cord blood T lymphocytes from preterm infants to gain insight into in utero mutational events. Mf determinations were made by using the HPRT T cell cloning assay on cord blood samples from 52 preterm infants. Natural logarithm Mfs (InMfs) from preterm infants were compared with results from our database for full-term infants. Our analysis revealed higher InMfs in cord blood T lymphocytes from preterm compared with full-term infants (P=0.008). In addition, preterm females had significantly higher InMfs compared with full-term females (P<0.001), whereas preterm males were found to have significantly lower InMfs than preterm females (P=0.005). Regression analyses also demonstrate a significant relationship between InMf and gestational age for preterm females that does not exist for preterm males. These results demonstrate the gender-specific association between Mf and age in humans.